1. Field of Invention.
This invention relates to spring armatures for "stored energy" printing devices and more particularly to spring armatures for dot matrix printers. In such stored energy printing devices, the spring armature is part of a magnetic circuit and is flexed into a retracted position by a permanent magnet attached to an actuator frame. In addition to a permanent magnet as part of each spring armature's actuating mechanism, a coil is also included. The coil generates an electromagnetic field to counteract the field of the permanent magnet, thereby releasing the spring armature to fly towards its impact position. When the spring armature is released, an impact tip at the end of the spring armature causes a dot to appear at the appropriate point on the print medium. As the current in the coil decays towards zero, the electromagnetic field also decays, allowing the spring armature to be retracted by the field from the permanent magnet.
Such dot matrix printers generally have a plurality of such spring armatures and associated actuator assembly attached to the actuator frame to form a print line. The actuator frame usually reciprocates in a direction parallel to the print line. Whenever a spring armature is located opposite to a point where a dot is desired as the actuator frame reciprocates, the appropriate coil is activated and a dot is printed.
2. Description of Prior Art.
In prior art designs of spring armatures, several conflicting design considerations have been taken into account. First, magnetically permeable materials are desired for increasing the flux carrying ability of the spring armature assembly. A highly permeable material used in the armature component of spring armature assemblies is typically a low carbon silicon iron or a low carbon steel; i.e. the carbon content of the steel or iron is less than 0.25% carbon. However, such magnetically permeable materials are generally not suitable for the spring component of the spring armature--for low carbon silicon iron and low carbon steel lack sufficient fatigue endurance to survive the repeated stresses of printing to form a satisfactory spring.
Second, the spring component of the spring armature (i.e. the component experiencing the maximum flexure when the assembly is cocked) should be made from a resilient material with a high fatigue endurance. Typically a high carbon steel or iron is used. However, high carbon steels are not as magnetically permeable as low carbon steels or iron. Thus, the same volume of a high carbon steel cannot carry as much magnetic flux as a low carbon steel.
One method for obtaining the benefits of both high carbon and low carbon steels or iron is to make the spring armature assembly from two separate components--for example, a leaf spring component is formed from a high carbon steel and a separate armature component is made from a magnetically permeable material. The spring component and the armature component are then joined together by welding or other common techniques to form an integral spring armature. Thus the armature is able to carry more magnetic flux, while still being resilient. An example of such a construction is disclosed in U.S. Pat. No. 4,351,235 to Bringhurst. However, a disadvantage of such two-component, integral spring armature assemblies is the joint between the two components is usually substantially weaker than either of the components and may cause reliability problems.
A second area of conflicting design considerations is the trade-off between having a thick cross sectional area to increase the flux carrying ability of the armature versus increasing printing speed. Thickening the cross sectional area of a spring armature will generally increase the mass of the spring armature; with increased mass incorporated in an otherwise identical actuator, the printing speed will be decreased. The decreased printing speed results from a slower acceleration of the spring armature towards the print medium and from a heightened tendency of the armature to rebound when it is retracted to the frame after printing on the media.
Also, thickened cross-sectional area exascerbates the stress in the spring portion. The thicker that spring portion is, the more difficult it is to bend, resulting in greater stress.
Another design choice is whether to place the impact tip near the end of the spring armature or near the center of percussion. Typically, the center of percussion--which is the point where the impact tip should be located to minimize the impulse generated onto the spring armature when the impact tip strikes the print media--is located remote from the unattached end of the spring armature U.S. Pat. No. 3,941,051 to Barrus et al. shows the impact tip being located near the center of percussion. If the impact tip is not located near the center of percussion, the greater impulse may cause the spring armature to rebound away from the frame after it is retracted from the print media. This rebounding slows printing speed Also, the greater impulse received by the spring armature increases the stress experienced by the spring armature reducing the useful lifetime of the spring.
However, in order to make the center of percussion coincide with the impact tip, additional weight has to be added beyond the impact point This greatly increases the effective mass of the spring armature and severly decreases the printing speed of the spring armature. Futhermore, locating the impact tip at the center of percussion of the spring armature limits the distance the impact tip may travel.
In addition, the resonant frequency of the spring portion of the spring armature acts as a maximum limit for the printing speed of the printer. As the printing speed approaches the resonant frequency of spring portion, the printer will cease printing properly.
Thus, it is an object of this invention to provide a unitary spring armature; i.e. the spring armature has no joints.
It is a further object to provide a resilient material in the spring region and a highly permeable material in the armature region while retaining the desired unitary structure.
It is yet a further object of the present invention to provide a spring armature with a minimal mass and with a high flux carrying capability.
Still a further object of the present invention is to provide a design where the impact tip need not be located near the center of percussion and to allow for faster printing.
It is an additional object to maximize the resonant frequency of the spring portion.